Drive shafts are commonly used in rear drive and four-wheel drive automobile and truck power trains and other vehicle drive trains to conduct torque and power from an engine or transmission located at one point in the vehicle to another site such as an axle assembly which can be rearward. Other applications include boating, farm or industrial power take-off shafts. Steel tubes have been used over the years and have generally proved reliable and effective. However, aluminum drive shafts offer advantages in weight and inertia savings if certain disadvantages in cost can be overcome.
One of the factors influencing the design of such a drive shaft is critical rotation speed, which is a function of the natural resonance of the shaft. If the critical speed of the drive shaft is within the rotary speed ranges encountered in normal use of the vehicle, especially under steady state conditions (e.g., cruising), such can result in noise vibration harshness (NVH) conditions. If the encounter with critical speed is transitory or brief, it may be tolerable whereas if it occurs at cruising speed, it is less tolerable. Increasing the drive shaft stiffness increases the resonance frequency and critical speed to such a high level that it is not encountered in normal cruising speeds which alleviates the NVH problem. An increase in diameter of the tube can stiffen the tube adequately but, in turn, can add weight unless the wall thickness of the tubing is made correspondingly thinner; but this, in turn, can require a higher strength material or a more costly composite.
One aluminum alloy that has been used for drive shaft applications is alloy 6061 which, according to the Aluminum Association (AA) registered limits, contains 0.8 to 1.2% Mg, 0.4 to 0.8% Si, 0.15 to 0.4% Cu, 0.04 to 0.35% Cr, the balance being aluminum and incidental elements and impurities. Some drive shafts made of 6061 are speed restrictable, and their use can involve a governor to reduce the drive shaft speed and avoid the drive shaft's critical speed. The use of longitudinal graphite fibers pultruded on the outside of a 6061 alloy drive shaft tube provides a composite drive shaft that is stiffer, such that it can spin or rotate faster, but this obviously adds to the cost of the drive shaft.
Another important aspect of a drive shaft is that it effectively transmits quite substantial amounts of torque. For instance, in a lightweight truck or a large automobile, the engine torque might be 350 pound feet at the engine crank shaft, but that can be elevated to a much higher level through a multiplying transmission (in a low gear) to a torque level of 1000 or even somewhat higher, for instance 1400 or 1500 pound feet of torque at the transmission output. Transmitting this torque obviously places a strength demand on the drive shaft in addition to the critical speed-vibration limitations. Aluminum drive shafts have been reported to make noises when the vehicle is placed into gear.
Typical drive shafts can range from about 2 or 3 inches in outer diameter (O.D.) to about 41/2 or 43/4 inches or even higher, especially for trucks, for instance up to about 5 inches O.D. or even more, such as from 51/2 to 71/2 inches or more. Typical wall thicknesses are within about 0.05 or 0.06 inch up to about 0.08 or 0.09 or 0.1 inch or even thicker, for instance up to about 0.13 or 0.14 inch or 0.16 or 0.17 inch or more, for instance 0.25 inch. A typical drive shaft for an automobile could have an O.D. of about 3.5 inches and a wall thickness of about 0.08 inch, whereas a drive shaft for a truck could have an O.D. of about 41/2 or 5 inches and a wall thickness of around 0.07 to 0.09 inch and a typical medium duty truck drive shaft can have an O.D. of 7 inches and a wall thickness of about 1/4 inch.
In addition to the performance demands on the drive shaft, the material selected for a drive shaft needs to be readily capable of the fabrication steps employed in making a drive shaft, which can include welding yokes at each end for universal joints which, in turn, requires that the material selected be weldable and that it have good strength capability after welding.
An object of the present invention is to provide an aluminum alloy drive shaft that is quiet during periods of high torque.